Study and Improvement on Measurement Accuracy of Image Based Overlay

The accuracy of overlay metrology becomes crucial when shrinking device dimensions to the 1X nm nodes and below. TIS (Tool Induced Shift) and WIS (Wafer Induced Shift) are the main issues for IBO (Image Based Overlay). Many factors could induce measured offset such as low signal contrast for opaque stack materials or mark profile degrade due to processes like CMP (chemical mechanical polishing). These optical measurement offsets will induce final device performance to degrade. In this paper, investigations of measured offset in IBO metrology induced by mark profile are presented, in the meanwhile, an analytical mechanics model is presented to provide insight into the connection between mark profiles and overlay measured offset. In the second part of the work, methods of compensation have been introduced via shift focus in the optical measurement system, and corresponding experimental results have been investigated as evidence to validate the IBO overlay results.